1. Field of the Invention
This invention relates to a layer with an embedded network of distributed sensors and actuators that can be surface-mounted on or embedded in a structure for monitoring its structural condition and for detecting anomalies in the hosting metallic or nonmetallic (composite) structures. In particular, the layer can be embedded in composite structures for monitoring curing progression and impact load and for detecting delamination failure and damage. The layer can also be surface-mounted on metallic structures for detecting delamination, crack generation and propagation.
2. Background of the Related Art
Nearly all in-service structures require some form of maintenance for monitoring their integrity and health condition to prolong their life span or to prevent catastrophic failure of these structures. Current schedule-driven inspection and maintenance techniques can be time-consuming, labor-intensive and expensive. The existing visual inspection techniques are often inadequate to identify a damage state invisible to the human eye, such as delaminations in composite structures. However, the current non-destructive evaluation techniques, such as ultrasonic and eddy current scanning, acoustic emission, and X-ray inspection, although useful for inspection of isolated locations, are impractical in many cases in terms of downtime and human involvement such as field inspection of in-service structures.
Recent advances in smart structure technologies and material/structural damage characterization combined with recent developments in sensors and actuators have resulted in a significant interest in developing new diagnostic techniques for in-situ characterization of material properties during manufacturing, for monitoring structural integrity, and for detection of damage to both existing and new structures in real time with minimum human involvement.
In order to develop such techniques, the structures considered must be equipped with a builtin network of a large array of sensors and actuators. There are currently no effective techniques for implementing a large array of sensors and actuators into existing or new structures economically and efficiently. Although piezoelectric materials have been considered for use as sensors and actuators, they are treated individually in installation and implementation. They must be individually placed on the hosting structures, leaving all the connection wires hanging out from the structures. Because of the way in which piezoelectric elements are installed, each element must be calibrated separately.
A system for in-situ delamination detection in composites is disclosed in U.S. Pat. No. 5,814,729, issued to Wu et al. The system consists of piezoelectric actuators and fiber optic sensors embedded within a laminated composite structure. The actuators generate vibration waves that propagate through the structure, and the sensors sense these strain waves, generating signals. Damping characteristics of the waves are calculated from the signals, and delamination regions can be determined. This system has a number of drawbacks. First, it is not designed for accommodation of a large array of piezoelectric actuators. Each sensor and actuator must be placed individually, leaving the wiring and calibration problems listed above unsolved. Second, the fiber optics are used as sensors only. The method of detecting damage, known as xe2x80x9cline-of-sightxe2x80x9d or xe2x80x9cthrough-transmission,xe2x80x9d requires the damage to lie exactly in the path of an actuator-sensor pair for it to be detected. Damage in surrounding regions cannot be detected using the system of Wu et al. Thus, the method of Wu et al. cannot be applied to the installation of a large sensor network, and uses only fiber optics for sensors and piezoelectric materials as actuators.
A self-contained conformal circuit for structural health monitoring and assessment is described in U.S. Pat. No. 5,184,516, issued to Blazic et al. The system consists of a series of stacked layers and traces for sensing strain and cracks in a surface. Flexible circuits are used to create the electrical interconnects. The entire structure is then affixed to the surface of a material being tested, after which testing occurs. This system has a number of limitations. It is only useful for monitoring conditions in the plane of the conformal circuit, i.e. the surface structure, and not interior structure of a laminated material. Information about internal damage, including delamination, cannot be obtained. In addition, the conformal circuit can only collect information at discrete points (i.e. the location of crack and strain gauges). The area between sensors cannot be monitored. Information about existing damage can also not be obtained. In addition, acoustic emission sensors are passive sensors-they indicate damage but cannot locate it without further information.
U.S. Pat. No. 5,869,189, issued to Hagood, IV et al., discloses composites for structural control that can be used for both actuating and sensing deformations. The composite includes a series of flexible, elongated piezoelectric fibers arranged in a planar, parallel array and separated by a relatively soft polymer. The composite can be embedded within a structural component, with the fibers extending along the length or width of the component. Multiple layers of the composite can be used to generate complex deformations. Because of the geometry of the fiber placement (i.e. parallel arrays), it is difficult to sense and locate deformations that occur only within a small region of the component. The fibers can extend along the entire length and width of the composite, and it is intended more for sensing larger scale bending and vibration than for detecting delamination or damage at a particular location.
There are no existing techniques available for efficiently and economically integrating a large networked array of sensors and actuators into existing or new structures for real-time monitoring of structural integrity and/or for detecting damage in the structures.
Accordingly, it is an object of the present invention to provide a layer containing a network of a large array of built-in sensors and actuators with an integral diagnostics capability for incorporation into both existing and new structures made of both metallic (such as aluminum alloys) or nonmetallic (fiber-reinforced composites) materials.
It is another object of the invention to provide a layer that is capable of monitoring the structural health of an object on an on-going basis without requiring disassembly and without the need to take equipment out of service.
It is an additional object of the invention to provide a diagnostic layer that can be calibrated before being embedded into a structure, so that minimal software changes are required after the structure is manufactured.
It is a further object of the invention to provide a diagnostic layer that allows for easy embedding of sensors and actuators into fiber-reinforced composite structures during manufacturing.
It is an additional object of the invention to provide a method of monitoring the curing process during manufacture of a laminate material, thereby saving time, money, and energy during laminate manufacturing.
It is a further object of the invention to provide a method for determining the force-time history of an impact on a structure.
Finally, it is an object of the invention to provide a technique for identifying an area of damage in a composite material having an integral diagnostic layer.
The invention provides the following advantages:
Real-time monitoring and reporting of structural conditions, saving in maintenance costs
Minimum human involvement in structural diagnostics, reducing labor and downtime
Automation and self-diagnostics, enhancing safety and reliability
These and other objects and advantages are attained by a diagnostic layer for incorporation into composite and metallic structures, including laminate structures. The layer contains sensors and actuators and is capable of diagnosing physical deformations or mechanical stress within the layer on a continual basis. When embedded in a laminate structure, the layer can also accurately determine the point at which the curing process of the laminate structure is complete.
The diagnostic layer is used to detect a structural condition of a material and includes a thin dielectric substrate, a plurality of sensors spatially distributed on the substrate, and a plurality of conductive elements in the substrate for electrically connecting the sensors to an output lead. The sensors are capable of generating electrical signals representative of a structural condition of the substrate, and are preferably piezoelectric sensors, which generate electrical signals in response to physical deformations of the sensors. The layer can also include at least one actuator, and preferably a plurality of actuators spatially distributed on the substrate and also connected to the output lead by the conductive elements. Preferably, the sensors and actuators are not distinct; piezoelectric materials act as both sensors and actuators.
The diagnostic layer may also be incorporated into a diagnostic system for detecting a structural condition. This condition may be the location and size of damage in a structure, location and force of an impact to the structure, or the quality of manufacturing of embedded structures such as composite materials, including the progression of curing of a laminate material. In addition to the layer, the system contains a signal receiver unit electrically coupled to the output lead for receiving output signals from the sensors. This coupling may be by wireless means. The system may also have a signal generating unit electrically connected to the output lead for providing an input signal to the actuators. Also included in the system is an interface unit in electrical communication with the signal receiver unit and, preferably, the signal generating unit. The interface unit preferably includes: a processor unit for processing data from the signal receiver unit to detect the structural condition; a control unit for controlling the input signal to the signal generating unit; and a memory unit for storing the data from the signal receiver unit.
The present invention also provides a method of detecting a change in the condition of a structure containing a diagnostic layer as described above. The method includes the following steps: providing a hosting structure containing the diagnostic layer; transmitting a first input signal to an actuator of the diagnostic layer through the output lead; receiving a first set of output signals from the sensors in response to the first input signal; at a later time, inputting a second signal to the actuator; receiving a second set of output signals from the sensors in response to the second signal; and analyzing the first set of output signals and the second set of output signals to determine a difference between the two. This difference represents the change in structural condition of the material, which may be a location and size of damage, or progression of curing. The output signals may also be processed to generate data representative of first and second structural conditions of the material. Subsequent nth signals may also be sent and received for a predetermined time to monitor further changes between an nth set of output signals and a prior set of output signals. The diagnostic layer may be inside a composite material, or it may be bonded to an external surface of metallic or composite materials.
A method for detecting a physical deformation of a structure, preferably the force and location of an impact to the material, is similar to the above method. The method has the steps of receiving a signal from at least one of the sensors, in which the signal represents physical deformation of the sensor; and processing the signal to generate data representing physical deformation of the material.
Finally, a method of curing a laminate composite is provided. The method includes the steps of: providing an uncured composite material having a diagnostic layer; subjecting the uncured composite material to an elevated temperature that initiates curing of the material; and monitoring changes in the condition of the diagnostic layer of the composite material until the condition of the diagnostic layer is substantially constant. Preferably, the diagnostic layer is as described above, and signals are sent to the actuators and received from the sensors. When the received signals are constant, the curing is complete.